[Stoves] The Blues
Ronal W. Larson
rongretlarson at comcast.net
Sun Jan 13 19:05:02 CST 2019
Andrew and list:
This just to pick up on a few of your words from below: "oxygen diffuses into the flame".
This is accurate for most combustion. But with most TLUDs, the gases usually diffuse into the (secondary) oxygen. The result is "wispiness". There is a big difference on which is the larger outer medium. But the initial part of TLUD flames, where the flame is very close to the secondary air "hole" looks identical to a gas jet entering a body of air (but that is an illusion). So for the usual TLUDs, your phrase should be that the "pyrolysis gas diffuses into the flame".
The flame photo from Dr. Olivier that I forwarded is the traditional - gas jets entering air. This is NOT what most of us working with TLUDs have experienced.
I need to emphasize also that with methane combustion (which almost all of us can visualize), there is a different chemistry going on from combusting CO and H2. The former has the same number of particles [3] before and after combustion. The latter sees a reduction in particle number [3 down to two]
[CH4 + 2 O2 > CO2 + 2 H2O ; 2H2+O2 > 2 H2O; 2 CO +O2 > 2 CO2]; I don't see a relationship here to colors, but I think this pressure drop helps with TLUDS (that are leaving char behind).
Ron
> On Jan 13, 2019, at 4:58 AM, Andrew Heggie <aj.heggie at gmail.com> wrote:
>
> On Sun, 13 Jan 2019 at 02:02, Ronal W. Larson <rongretlarson at comcast.net> wrote:
>
>> I see the word "Gasifiers" here a lot and want to emphasize instead the word "pyrolysis". This list has a sister group called "gasification" - that never has any discussion of making charcoal - only of consuming it.
>
> The gasification list has dwindled worse than stoves, there have been
> no posts since November.
>
> Anyway stoves isn't only about TLUD, so gasification of whole wood
> happens in many stoves.
>
> I hope we can get more insight into this business of how rice hull
> burners have a blue flame which only seems to happen when burning wood
> if the gases and air are well mixed. The paper Alex linked to suggests
> potassium has an effect in burning sooty particles, interestingly it
> does point out that the amorphous parts burn out earlier than the
> graphitic bits, which is what I would have expected as graphite
> resists oxidation.
>
> It doesn't help with whether the higher ash content in rice husks
> causes some interaction that leads to a bluer flame.
>
> I have a new wood burning room heater that has an insulated firebox
> and a glass window. Once up to temperature and turned down it acts as
> a pyrolyser and the flames are a lazy purple blue floating above the
> hot wood, unfortunately my camera does not record what my eyes see
> otherwise I would post a picture of the secondary air inlets combining
> with the offgas.
>
> I have never seen any rice husks so cannot comment from personal experience.
>
> I understand how this happens in wood burning in that, when the flame
> is diffuse, the burning takes place in two stages, firstly the
> offgases dissociate and oxygen diffuses into the flame and combines
> with hydrogen, then the carbon rich remainder rises in the heat and
> radiates the characteristic yellow until it too combines with oxygen.
> When the gases are premixed the reaction is more instantaneous and the
> flame colour is bluer, often with a tinge of purple, which is more
> characteristic of CO, H2 and potassium.
>
> You can often see this at the base of a candle flame, it is bluer and
> if the wick shortens the flame becomes bluer.
>
> Andrew
>
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